In automotive networking, the CAN protocol has been used for some years. There, communication is event-triggered. Very high loads can be generated when the transmission of different messages is to be initiated at the same time. The non-destructive arbitrating mechanism of CAN guarantees that the sequential transmission of all messages according to the priority of their identifiers. For hard real-time systems, an analysis of the transit times and bus loads has to be done previously for the entire system to ensure that all message deadlines can be met (even during peak load).
There are already communication protocols which work on the basis of time-triggered processing, such as TTP/C or Interbus-S. In these protocols the bus access is scheduled already in advance by assigning transmission points. Therefore, no collisions can occur during transit time. However, a peak load on the communication bus is avoided as well. Thus, the bus is frequently not completely utilized to capacity in the process.
Time-determined communication systems permit a predeterminable execution of the processes. Therefore, it is possible for the application to be synchronized with the communication in a narrower time range than in an arbitrating system. In the case of a communication which is performed purely cyclically, however, the application is rigidly bound to the time pattern whereas certain operational sequences, in particular, the engine control in a vehicle, require a time-variable transmission of messages or data, in particular, of the measured values, which is adapted to the rotational speed.
These methods and configurations cannot provide optimum results in every respect.